I have been monitoring POI development for Europe and Germany for fun. I use the POI exports provided by cloudmade and count some selected POI's and look at the development over time. Unfortunately, cloudmade does not provide any POI exports for Europe anymore, so new data only contains German POI development. Interestingly, there is a trend of fewer and fewer POI additions recently. It could be possible that the German community gets tired of mapping, but I don't think that this is the case. I hope my stats don't have errors, but is it possible that most POIs in well mapped areas have been mapped and we are seeing a saturation effect? This would be great news and indicates that OSM reaches a new level of maturity.

Well but if I look at my own activities, I'm mostly doing aerial imagery tracing for now. So it would be interesting to compare this graph with building=yes history. Peter Körner releases Full history planet extracts, that might be used for your analyses